BOSTON -- The plan was for Edward Mujica to enter in the ninth inning with a comfortable 6-0 lead.

Though he has struggled early this season, Mujica has been tinkering with his delivery, and Red Sox manager John Farrell intimated on Saturday morning that the team was looking to use Mujica in low-leverage situations.

Saturday afternoon's game with the A's represented an ideal time for his next appearance.

During Mujica's warmup, however, he felt some oblique tightness. As a result, his day was over before it even started.

Mujica explained after the game that he threw a changeup in the Red Sox bullpen and felt a "little pinch" on the right side of his back. After another pitch, the feeling remained, something Mujica said he had never felt before.

The righty reliever informed bullpen coach Dana LeVangie, and the Red Sox dugout was told that Mujica would not be available for the ninth.

"When I start warming up, I didn't feel nothing," Mujica said after Boston's 6-3 win. "One pitch, I threw a changeup, I just felt it a little bit. After that, two times in a row, and I saw something's wrong here. I feel that thing for the first time. I talk to the bullpen coach and I tell him, and [he] makes a phone call, [to say] I don't feel comfortable."

"We’ll check him when he comes in tomorrow," Farrell said. "He’ll get treatment today and then we’ll re-examine him in the morning."

The injury had a domino effect on the rest of the Red Sox bullpen. Chris Capuano had been warming in the eighth inning in case Lester needed to be pulled, but was sat down when the inning was over. Then he was called upon to pitch the ninth.

The next hitter, Josh Donaldson, was hit by a pitch. After a first-pitch ball to Yoenis Cespedes, Farrell quickly yanked his lefty reliever.

Koji Uehara entered to clean up the mess but walked Cespedes. A David Ross throwing error allowed Donaldson to score, but Uehara got the game's final three outs to limit the damage to three runs.

For Capuano, the two runs he allowed were his first all season. In his first season as a reliever, Capuano had been close to perfect before Saturday. He hadn't allowed a run in 15 innings.

"Well, when you look at Chris who’s been so good for us, in a matter of seven or eight pitches there’s two runs on the board and another guy standing on base and things were happening quick. That’s a very good ream, utmost respect for them. They can strike quick. And fortunately Koji was able to come in and close things out."

Farrell said the nature of Capuano's bullpen work may have contributed to his subpar performance.

"We had him up to close out the eighth if Jon’s pitch count climbed any further," Farrell said. "And then trying to get Edward Mujica another inning of work today, [we] sat Chris down. And then while warming up, Edward felt a little bit of an oblique tightness so then we had to turn back on Capuano.

"I think the on-off-on again -- I don’t know if it caught [Capuano] by surprise, but today’s outing has been uncharacteristic with what Chris has done all year for us."

Farrell said that with a pitch count that had climbed to 119 pitches, there was no real thought given to bringing Lester back out for the ninth inning.

"No, we’re still in the early stages of this season," Farrell said. "He had thrown 122 pitches his last outing, even though an extra day of rest today, he’s probably going to get an extra day or two this next turn. But [we] felt like that game, six-run lead, we’re completely confident in the bullpen to close things out."

Phil Perry serves as a general reporter for Comcast SportsNet, mainly covering the New England Patriots. Follow Phil on Twitter here.